The present invention relates to a circuit for use in photographic cameras, and more particularly, to an exposure information setting circuit which produces a signal indicative of shutter speed upon receipt of signal indicative of the brightness of the object to be photographed.
In photography it is necessary to set the camera at the proper shutter speed and F number or aperture setting with respect to a preselected ASA or DIN number. These three factors, i.e., shutter speed, F number and ASA number are determined by the brightness of the object to be photographed. Since each factor is variable, it is photographer's choice to set these three factors to his taste. If, however, two of the factors are fixed, the remaining factor will be determined by the brightness of the object. In an automatic exposure control system for use in single-lens reflex camera employing through the lens (TTL) system, there have been employed an exposure information setting circuits which operate to calculate a suitable exposure time Tv in accordance with the so-called APEX system given by the following equation. EQU Tv=Bv-(Av-Sv)=Bv+Sv-Av (1)
wherein Bv is the given value of objective brightness, i.e., the brightness of the scene to be photographed, Sv is the film sensitivity value and Av is the aperture value. These values Bv, Sv and Av are referred to as APEX indices hereinbelow.
In FIG. 1, there is shown an exposure information setting circuit which includes a photodiode PD.sub.2 and diode D.sub.1 connected in series across a first power source E.sub.1. When the photodiode PD.sub.2 receives light from the object to be photographed, a voltage signal V.sub.B which is related to the exponential value of the brightness is produced across the diode D.sub.1. Here, the voltage signal V.sub.B is identical with the APEX index Bv of brightness value given in the equation (1). The principle circuit further includes potentiometers PM.sub.3 and PM.sub.4 which are parallel to one another and are connected to a second power source E.sub.2. A sliding arm or wiper W.sub.3 of the potentiometer PM.sub.3 slides along the potentiometer to be set in a position corresponding to film sensitivity and is connected to junction P.sub.2 between photodiode PD.sub.2 and diode D.sub.1. A wiper W.sub.4 of the potentiometer PM.sub.4 slides along the potentiometer to be set in a position corresponding to aperture value and is connected to an output terminal P.sub.4. In this circuit the voltage drop V.sub.S between the wiper W.sub.3 and a junction P.sub.3 of potentiometers PM.sub.3 and PM.sub.4 is identical with the APEX index Sv of film sensitivity value and the voltage drop V.sub.A between the wiper W.sub.4, and the junction P.sub.3 is identical with the APEX index Av of aperture value.
According to the circuit described above, the voltage V.sub.T which appears across ground line P.sub.1 and output terminal P.sub.4 can be given as; EQU V.sub.T =V.sub.B +V.sub.S -V.sub.A ( 2)
in other words, the circuit performs a calculation corresponding to the solution of the above described equation (1) showing APEX system.
Identical results can be obtained in the case where the output P.sub.4 of the circuit shown in FIG. 1 is taken in reference to a junction P.sub.3 ' of the negative terminal of the power source E.sub.2 and potentiometers PM.sub.3 and PM.sub.4. In this case, the voltage drop V.sub.S ' between the wiper W.sub.3 and the junction P.sub.3 ' corresponds to the APEX index Sv of film sensitivity value, while the voltage drop V.sub.A ' between the wiper W.sub.4 and the junction P.sub.3 ' corresponds to the APEX index Av of aperture value, thus, the output V.sub.T of the circuit can be given as; EQU V.sub.T =V.sub.B -V.sub.S '+V.sub.A ' (3)
the circuit of FIG. 1 employs two power sources E.sub.1 and E.sub.2. However, it is undesirable for the camera to have power sources occupy much space in the limited space available in the camera. Accordingly, there has been proposed a circuit which has the same function as that described above while employing only one power source. One example of such a circuit is shown in FIG. 2 in which the input voltage V.sub.B produced across the diode D.sub.1 is supplied to a noninverting input of an operational amplifier A.sub.1. The output terminal of the operational amplifier A.sub.1 is connected to one junction P.sub.7 of potentiometers PM.sub.5 and PM.sub.6 which are connected in parallel with each other. The other junction P.sub.8 of the potentiometers PM.sub.5 and PM.sub.6 is connected to ground through series connection of a transistor Q.sub.12 and an adjusting resistor R.sub.11. The transistor Q.sub.12 has its collector connected to the junction P.sub.8 and its emitter connected to the adjusting resistor R.sub.11. The base of the transistor Q.sub.12 is connected to a transistor Q.sub.11 at its collector. The transistor Q.sub.11 has its base and emitter connected to each other to function as a diode, has the emitter connected to ground and has the collector connected to the positive side of a power source such as E.sub.1 through a suitable resistor R.sub.10. These transistors Q.sub.11 and Q.sub.12 and adjusting resistor R.sub.11 are connected to form a known type of constant current generator. The wiper W.sub.5 of the potentiometer PM.sub.5, provided for establishing voltage signal V.sub.S indicative of the APEX index Sv of film sensitivity value, is connected to the inverting input of the operational amplifier A.sub.1 to form a negative feedback circuit, so that the voltage drop appearing between the input terminals P.sub.5 and P.sub.6 is approximately maintained at 0 (volt). Accordingly, the signal appearing at the wiper W.sub.5 is approximately equal to the signal appearing at the noninverting input P.sub.5, that is, the signal V.sub.B. The wiper W.sub.6 of the potentiometer PM.sub.6, provided for establishing voltage signal V.sub.A indicative of the APEX index Av of aperture value, is connected to an output terminal P.sub.9. As a result, the voltage between the ground and the output terminal P.sub.9 is (V.sub.B +V.sub.S -V.sub.A) which is equal to the above mentioned value of output V.sub.T indicative of exposure time. Thus, by the employment of the operational amplifier A.sub.1 and transistors Q.sub.11 and Q.sub.12, the circuit of FIG. 2 permits calculation of exposure time without requiring a supplementary power source, such as power source E.sub.2. Further information for this type of circuit shown in FIG. 2 is described in detail in U.S. Pat. No. 2,936,842 of Nanba et al or U.S. Pat. No. 3,977,011 of Matsuda.
However, according to this type of circuit as described above, it is inevitable to provide the adjusting resistor R.sub.11 for the reasons described hereinbelow.
In an operational circuit such as shown in FIG. 2 or the circuit of FIG. 1, presuming an ambient temperature of 25.degree. C., the value of the voltage change which corresponds to a 1-step change of the APEX index value is required to be 18 mv. This value is determined in reference to the current-voltage characteristics of the semiconductor employed as a logarithmic conversion element. In a diode, for example, a two-fold change in current therethrough normally corresponds to a change of 18 mv across the terminals thereof, and in a transistor a two-fold change in collector current corresponds to an 18 mv change in base-emitter voltage. Thus, in FIG. 1, when there is a two-fold change in the amount of light incident on photodiode PD.sub.2, i.e., when there is a 1-step change of APEX index value, there is a two-fold change in photocurrent, and hence an 18 mv change in voltage V.sub.B across the terminals of diode D.sub.1. The potentiometer circuit in the right-hand portion of FIG. 1 or FIG. 2 is required to accurately respond to input signals and accurately produce output signals that change 18 mv for each 1-step change in APEX value.
These requirements can be met in closely controlled manufacturing conditions. However, at present, such requirements can not be met in large-scale production. In other words, if it were possible to easily obtain, in large quantity, transistors Q.sub.11 or Q.sub.12 in which the requisite value of collector current could be obtained without need of adjustment, or if it were possible to easily obtain potentiometers having required resistance characteristic curves, there would be no problems. In large-scale production ensuring accuracy of values of circuit components beyond a certain point is considered too costly, and it is a current practice to tolerate a variation of 20% above or below the designed values of the characteristics of components. In the circuit of FIG. 2, therefore, in order to accurately obtain a signal voltage of 18 mv corresponding to a 1-step change in exposure time information, it is necessary to adjust the current which flows in potentiometers PM.sub.5 and PM.sub.6. To achieve this, it is necessary to add adjusting resistor R.sub.11 to the exposure control circuit, which for the most part is constituted as an integrated circuit module. Because of this necessary addition of an extra resistor there is the disadvantage that extra work for positioning wiring, and adjustment is necessary, and hence the cost of production of the exposure control circuits is increased.